Crate no_panic

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A Rust attribute macro to require that the compiler prove a function can’t ever panic.

no-panic = "0.1"
use no_panic::no_panic;

fn demo(s: &str) -> &str {

fn main() {
    println!("{}", demo("input string"));

If the function does panic (or the compiler fails to prove that the function cannot panic), the program fails to compile with a linker error that identifies the function name. Let’s trigger that by passing a string that cannot be sliced at the first byte:

fn main() {
    println!("{}", demo("\u{1f980}input string"));
   Compiling no-panic-demo v0.0.1
error: linking with `cc` failed: exit code: 1
  = note: /no-panic-demo/target/release/deps/no_panic_demo-7170785b672ae322.no_p In function `_$LT$no_pani
3002b8d9fE+0x2): undefined reference to `

          ERROR[no-panic]: detected panic in function `demo`
          collect2: error: ld returned 1 exit status

The error is not stellar but notice the ERROR[no-panic] part at the end that provides the name of the offending function.


  • Functions that require some amount of optimization to prove that they do not panic may no longer compile in debug mode after being marked #[no_panic].

  • Panic detection happens at link time across the entire dependency graph, so any Cargo commands that do not invoke a linker will not trigger panic detection. This includes cargo build of library crates and cargo check of binary and library crates.

  • The attribute is useless in code built with panic = "abort".

If you find that code requires optimization to pass #[no_panic], either make no-panic an optional dependency that you only enable in release builds, or add a section like the following to your Cargo.toml or .cargo/config.toml to enable very basic optimization in debug builds.

opt-level = 1

If the code that you need to prove isn’t panicking makes function calls to non-generic non-inline functions from a different crate, you may need thin LTO enabled for the linker to deduce those do not panic.

lto = "thin"

If thin LTO isn’t cutting it, the next thing to try would be fat LTO with a single codegen unit:

lto = "fat"
codegen-units = 1

If you want no_panic to just assume that some function you call doesn’t panic, and get Undefined Behavior if it does at runtime, see dtolnay/no-panic#16; try wrapping that call in an unsafe extern "C" wrapper.


The linker error technique is based on Kixunil’s crate dont_panic. Check out that crate for other convenient ways to require absence of panics.

Attribute Macros§